how to replace a carburetor

How to Replace a Carburetor

Did a family member donate an old leaf blower to you which “just needs a little elbow grease?” Did you make a good find at a local garage sale? Have you just neglected the old lawn mower you’ve had collecting dust in the garage? In any case, it’s likely you’re visiting this page because you can’t get your engine driven equipment to start up or have reason to believe your carb needs replacing. This guide may document my replacement of the carb in my leaf blower, but the information is relevant for working with any engine which has a carburetor.

What are the symptoms of a bad carburetor?

Before I go over how to replace the carburetor, I want to go over the symptoms of a faulty carburetor. If your leaf blower (or any engine with a carburetor) is experiencing the following, it may be time to replace, or at least clean the carburetor:

Dark smoke coming from engine exhaust - a lot of dark smoke is an indicator that the fuel-to-air ratio (which is controlled by the carburetor) is a little too “fuel-heavy”.
Engine stalling - if the engine stalls out, especially while idle, it could mean that the faulty carburetor is starving the engine of fuel.
Trouble starting - carburetor problems are going to become especially clear when trying to perform a cold start. An engine is going to be even more sensitive to the fuel-to-air ratio while trying to start cold.

How does a carburetor work?

To demonstrate most simply, think of a carburetor as a tank which has a floating ball in it. At the top of the tank there is a hole smaller than the ball. This hole is where liquid enters the tank, so as the tank fills with liquid, the ball blocks the hole and the tank stops filling with liquid. Now the bigger difference in size between the ball and the hole, the more air there will be in the tank compared to the liquid. See where I’m going with this?

In this analogy, the liquid is gas, the ball is our float valve, and the tank is our carburetor. The carb I will be replacing in this article is from a leaf blower, but for demonstrating the function of a carb, I think this old carb from a snow blower in the following picture does a better job.

As you can see, there is a reservoir on the bottom of the carb which houses the float valve. As the reservoir fills with fuel, the float valve shuts and stops any more fuel from entering the reservoir. As the engine sucks in fuel/air, the fuel from the reservoir will enter the carb through a nozzle. At the same time, air will enter the carburetor after passing through the air filter. This mixture of fuel and air only exists in the carb momentarily before going into the engine cylinder to be combusted.

To go one step further, you may also notice two butterfly valves in the carburetor. These valves will have holes in them as to not completely seal off the carb chamber, when closed. These valves have two names: the throttle valve and the choke valve. In case the names don’t explain their function enough, I’ll do my best:

Throttle valve - This valve opens as the engine is throttled via a gas pedal or trigger, introducing more fuel to the engine and therefore outputting more power. The throttle valve is not visible in the pictures above, but it is located directly behind the choke valve.
Choke valve - This valve also enriches the fuel-to-air ratio in the carb, but it is used to get the engine started when cold. When the engine is cold, the gas will not vaporize as easily, so the choke valve works to lower the pressure in the carb which allows the gas to vaporize. Vaporizing is the same as boiling and as pressure decreases, so does the boiling point of a fluid.

How do you remove the carburetor?

Once you’ve honed in on the carburetor as the bad actor, you are ready for disassembly. The maybe not-so-obvious topic I may have to cover here is… how do you find the carburetor. The answer is usually pretty simple: Look for the air filter. The air filter is typically attached directly to the carb, although you may have to remove a housing or casing to get to the air filter. Tip: Take lots of pictures as you take anything apart. It really helps with the whole “putting it back together” part. So let’s take a look with what we got going on with my leaf blower to the right.

After locating the air filter, remove it, and examine the carburetor to see how it is attached to the engine intake. You will likely need a screwdriver or maybe even a wrench to unfix the carb. Also, you will need to remove the fuel hose, a fuel overflow hose it has one, and/or another hose if the carb has a nonintegral priming bulb. Finally, there will likely be a thin metal rod connected to the carb which controls the throttle valve, simply unhook this rod. Now the carb should be able to be removed. Once the carb is removed, I will usually stick a rag or paper towel in the engine intake while I continue to work just to stop any debri from entering the engine.

This is a very important part in the process. Before you go out buy a new carb, examine it. If it looks to be in good shape aside from dirt and gunk, you may be able to just clean and reinstall it. They make “carb cleaner” which is specifically made for this application and should be available anywhere you can buy motor oil. But for now, let's assume your carb is fubar, and you already have a new carburetor suitable for your machine (in this case - a leaf blower).

How do you install a new carburetor?

Most of the time when you buy a new carb, it will come with new gaskets, fuel hoses, fuel filter, priming bulb, and who knows what else. I’d recommend replacing everything new, because if you don’t, you’re just going to end up losing these other new parts before you ever need them again. To install these new components, you pretty much just have to do everything you did to remove the carburetor, just in reverse.

In addition to the gasket seals, the fuel hose connection to the fuel tank is another location prone to leaks, so I would highly recommend replacing the fuel hoses and stopple as these components are made of rubber and usually stored outside which leaves them prone to degradation. In my case, the rubber stopple had actually come completely loose and my leaf blower would leak fuel with any sudden movements.

To the right, you can see the entire replacement kit that I received.

Here are some final pictures from my little project. Hopefully, you were able to learn a little about carburetors by reading this. They may seem like complicated mechanisms at first, but once you learn a bit about how they work, they really become a relatively easy thing to mess around with. Good luck, get dirty, and don’t hurt yourself!

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